Progesterone plays an important role in the establishment and maintenance of pregnancy. However, there have been no systematic reports on the expression of the rat uterine progesterone receptor (PR) gene during this process. In the present studies PR mRNA levels in rat uterus were analyzed during the estrous cycle and early pregnancy (d1-d10), using reverse transcription-polymerase chain reaction (RT-PCR). PR mRNA level was the highest at estrus and the lowest at metestrus, with the former value twice the latter. Post-coitum PR transcription increased rapidly, reached a peak of two fold of the d1 level on d3-d4 (before implantation), but drastically dropped to 50% of the d1 level on d6 (during implantation) and remained unchanged until d10. It is of interest to discover that the uterine PR mRNA level at the implantational sites was much higher than that at the non-implantational sites on d7-d10. Preliminary discussion was made on the physiological significance of these changes.